1. Field of the Invention
The invention relates to belt tensioning devices and arrangements, and in particular to a hydraulically actuated device for tensioning the endless drive belt of the drive system for vehicle accessories. More particularly, the invention relates to a belt tensioner having a self-contained hydraulic fluid supply-idler pulley-pump assembly driven by the endless belt, which maintains a predetermined constant tensioning force an the endless drive belt regardless of the engine operating condition.
2. Description of the Prior Art
There is the trend today in the automobile industry to operate the various vehicle accessories, such as the power steering pump, oil and air pumps, air conditioning and alternator, by a single endless belt driven by a pulley connected to the engine crankshaft. This system is referred to as a "serpentine" drive belt system. To insure optimum operating efficiency for these various accessories, it is necessary that the drive belt be maintained at a predetermined tension to assure efficient performance of the accessories as well as satisfactory service life for the belt. Due to the relatively greater length for the single drive belt which replaces the heretofore plurality of smaller belts, there is a greater tendency for the belt to stretch, which will affect the operating characteristics of the driven accessories. Therefore, it is desirable that an automatic belt tensioning device be used for these endless belts to provide reliable service over an extended period of time and to maintain a constant amount of tension thereof regardless of the amount of belt stretch without requiring any maintenance or manual adjustment.
Numerous devices have been proposed and used to accomplish this purpose. One type of tensioner uses a bushing formed of an elastomeric material which is placed in compression by some mechanical means for continuously exerting a tensioning force on the belt. Examples of these constructions are shown in U.S. Pat. Nos. 3,975,965 and 4,144,772. These tensioner constructions, which use an elastomeric material, have the disadvantages in that the high load rate which they exert on the belt results in the rapid loss of tensioning as the belt stretches, and this load rate limits the stroke of the belt-engaged idler pulley to a shorter distance than desired. Also, sudden acceleration and deceleration of the drive belt can cause a whipping action to occur which creates a time lag before full damping is achieved.
Numerous other types of belt tensioning devices use springs for applying and/or maintaining the tensioning force on a belt-engaging idler pulley or chain-engaging sprocket. Some examples of these types of constructions are shown in U.S. Pat. Nos. 2,703,019, 2,893,255, 3,413,866, 3,631,734, 3,768,324, 3,812,733, 3,924,483 and 3,965,768. Some of these various spring-actuated devices use the biasing force of the spring in combination with hydraulic actuated members for regulating the amount of tensioning force applied to the belt, depending on whether the engine is running or shut off.
U.S. Pat. No. 2,051,488 shows a chain tensioning device in which a hydraulic cylinder is actuated when the engine is running to reduce the tensioning pressure which is exerted on the drive chain by a biasing spring. This hydraulic cylinder applies a counter force to the biasing action of the spring when the engine is running in order to decrease the tensioning force on the chain, whereupon the full biasing force of the spring is exerted on the chain when the engine is off and the hydraulic cylinder inactive. U.S. Pat. No. 3,142,193 discloses another belt tensioner using a hydraulically actuated bellcrank and belt-engaging pulley in which a spring biases the pulley into tensioning engagement with the belt until the engine is operating, whereupon the hydraulic actuated piston imparts a greater tensioning force to the pulley through the bellcrank. When the engine stops, the piston retracts and the spring maintains a lesser tensioning force on the belt.
U.S. Pat. No. 4,077,272 discloses another belt tensioner using both a spring and a hydraulic member to achieve the desired tensioning characteristics. In this device, when the engine is running, hydraulic oil from the engine lubricating system forces a piston back into its cylinder to overcome an internally mounted first spring so that only a second spring acts on the belt tensioning pulley when the engine is operating. When the engine is off, both springs act on the belt tensioning pulley. U.S. Pat. No. 3,132,596 shows a belt tensioner using a hydrostatic snubber controlled by the power steering pump of the vehicle, whereupon the snubber is forced outwardly in direct relationship to the output pressure of the power steering pump. This mechanism does not maintain a constant pressure on the belt when the vehicle engine is both on and off, and will apply different forces in relationship to the engine speed.
Many of these devices are believed to perform satisfactorily for their intended purpose. However, the use of springs for applying the tensioning force on a drive belt or chain, either for applying or retracting a tensioning member therefrom, presents problems. The operating characteristics of these springs will change over the life of the spring and even in response to changes in ambient temperature. Also, as the drive belt stretches, the spring's biasing effect changes, making it difficult to maintain a constant tensioning force on the drive belt. Likewise, the springs may become rusted and corroded and break during the life of the vehicle, presenting mechanical maintenance problems for the vehicle owner.
Many of these problems are believed to be eliminated by the improved hydraulic belt tensioner constructions shown in a copending application of Nolte V. Sproul, Ser. No. 058,362, filed July 18, 1979, and my copending application, Ser. No. 060,903, filed July 26, 1979, both assigned to the same assignee as is the present application. These improved belt tensioners are operated by hydraulic pressure exerted by fluid from the vehicle engine, such as the relatively low fluid pressure of the engine's oil lubricating system or the high fluid pressure developed by the power steering pump of the vehicle.
However, some vehicle manufacturers prefer not to use the hydraulic fluid which is used for other purposes in the engine, such as lubricating or power steering, for the operation of the belt tensioner. This objection is believed to be eliminated by providing a hydraulic belt tensioner having its own self-contained hydraulic fluid reservoir and supply, such as shown in my copending application, Ser. No. 063,205, filed Aug. 3, 1979. This improved tensioner operates in conjunction with a pivotally mounted accessory for tensioning the drive belt. Again, some vehicle manufacturers desire a belt tensioning device which can be mounted as a complete unit on the engine and automatically tension the drive belt without the need of pivotally mounting one of the vehicle accessories. This provides a certain amount of flexibility in engine design since the tensioning unit may be mounted at various locations for engaging the belt and does not rely on the pivotal mounting of one of the accessories.
Accordingly, it is desirable to provide a belt tensioning device and arrangement which eliminates the use of springs for controlling the belt tensioning force, which device will maintain a constant predetermined tension on the belt throughout the belt life and regardless of whether the engine is on or off or being driven at high or low speeds, which device uses a source of hydraulic fluid completely independent of the vehicle hydraulic system for its operation, and which does not require movement of one of the vehicle accessories for tensioning the drive belt which is operatively engaged with said accessory.
There is no known belt tensioning device or arrangement of which I am aware which imparts and maintains a nearly constant predetermined tensioning force on an endless drive belt by a hydraulic piston which is actuated by hydraulic fluid from a self-contained fluid supply reservoir completely independent of the vehicle hydraulic fluid system, which maintains this constant pressure on the belt whether the engine is on or off or operating at various speeds, which prevents belt whip and achieves a highly efficient damping effect, and which does not require movement of one of the vehicle accessories to achieve the tensioning effect.